_why?

One language to rule them all. JavaScript is everywhere, thus so is CoffeeScript. Servers, browsers, even databases. If extending this to rendering logic and UI structure (server and client side) is desirable to you, CoffeeKup is your friend.

More specifically, one outstanding language. CoffeeScript is one hell of a clean, expressive, flexible and powerful language. It's hard to find such combination, especially if you need it to run in the browser too.

Not yet another specialized language to learn. Transferable knowledge FTW.

Embed your templates in CoffeeScript nicely. Templates are just functions, so they don't lose syntax highlighting and syntax checking when embedded in CoffeeScript apps.

Embed CoffeeScript in your templates nicely. In the same manner, you can write the contents of <script> blocks in CoffeeScript, and keep the highlighting. Perhaps more significantly, the CoffeeScript compiler doesn't have to be called just to convert these blocks to JS, as in other templating engines.